The invention relates to a controllable energy storage device and a method for operating a controllable energy storage device.
It has become apparent that in future both in the case of stationary applications, for example wind turbines, and also in vehicles, for example hybrid vehicles or electric vehicles, electronic systems are being used ever more frequently, which electronic systems combine new energy storage technologies with electrical drive technology. In conventional applications an electrical machine, which is embodied for example as a three-phase machine, is controlled by way of a converter in the form of an AC converter. A characterizing feature of systems of this type is a so-called DC voltage intermediate circuit, by means of which an energy storage device, generally a battery, is connected to the DC voltage side of the AC converter. In order to be able to fulfill the particular requirements relating to power and energy for a particular application, a plurality of battery cells are connected in series. Since it is necessary for the current that is provided by an energy storage device of this type to flow through all the battery cells and a battery cell can only carry a limited amount of current, battery cells are often in addition connected in parallel in order to increase the maximum current.
When connecting a plurality of cells in series, the problem arises in addition to a high total voltage that the entire energy storage device fails if a single battery cell fails because it is then no longer possible for the current to flow through the battery. A failure of this type of the energy storage device can cause the entire system to fail. In the case of a vehicle, a failure of the drive battery causes the vehicle “to be immobilized”. In other applications, for example in the case of a rotor blade adjustment in wind turbines, situations that pose a safety risk can even occur in unfavorable boundary conditions, for example in the case of a strong wind. Efforts are therefore continuously being made to achieve a high level of reliability of the energy storage device, wherein the term “reliability” describes the ability of a system to function in a fault-free manner for a predetermined period of time.
U.S. Pat. No. 5,642,275 A discloses a multi-level convertor having at least one phase, wherein each phase comprises a plurality of full bridge invertors having in each case allocated independent DC sources.